1. Field of the Invention
This invention relates to a method and apparatus for continuing treatment of a liquid inventory in order to remove a selected ingredient from the inventory. More particularly the invention concerns a method and apparatus for continuing removal of PCBs (polychlorinated biphenyls) from a transformer in which a PCB containing transformer fluid has been replaced with a non-PCB transformer fluid.
2. Description of the Prior Art
Many transformers were manufactured with 40-70 weight percent PCBs (polychlorinated biphenyls) in trichlorobenzene solvent ("askerel" fluid) as the transformer fluid which functions as a coolant/insulator. Recognition of the hazards of PCB has resulted in laws and regulations which (a) ban use of PCB as a transformer fluid in new transformers; (b) require removal of PCB transformers fluid in certain applications such as food processing applications and replacement of the PCB transformer fluid with a non-PCB fluid such as silicone; and (c) require that treated transformers maintain a low PCB content in the transformer fluid after a 90 day period following the treatment, e.g., less than 50 parts of PCB by weight per million parts of fluid to be classified as "non-PCB". Another "PCB-contaminated" transformer classification permits up to 500 ppm PCB content in the transformer fluid.
If the PCB concentration in retrofilled transformer fluid tests less than 50 ppm or 500 ppm after the required waiting period, the transformer qualifies for reclassification and continued use. Within 2 to 3 years following reclassification, the PCB concentration (initially less than 50 ppm) may rise to 250 to 300 ppm as a result of gradual leaching from wooden components and interior surfaces, or may rise over 500 ppm in transformers which originally test below that level. A typical state-of-art resolution is to replace some or all of the transformer fluid with fresh silicone oil to re-establish an acceptable reclassification concentration. Such replacements may be carried out several times. If the transformer had been retrofilled with a chlorinated solvent instead of silicone, distillation equipment may be brought to the transformer site to effect the desired reduction in PCB level.
A service industry has developed for replacing PCB transformers fluids with silicone fluids. See U.S. Pat. No. 4,124,834 which describes the replacement of PCB transformer fluid with silicone oil and recognizes that there is ongoing leakage of residual PCB (from the fibrous and solid insulating materials in the transformer) into the silicone oil. The patent further describes a treatment vessel which continuously treats a side stream of the transformer fluid inventory with activated carbon in an external housing by moving the side stream from the transformer tank through an activated carbon bed to permit the adsorption of PCB on the activated carbon. The treated silicone fluid is returned to the transformer tank. Movement of the fluid may result from thermal siphon phenomena or from a pump in the fluid piping. As a result of this operation, the PCB content of the silicone transformer fluid in the retrofilled transformer can be maintained below the maximum value permitted by law and regulations.
It may be desirable to continue treatment of the transformer fluid for many years because of the continuing gradual leaching and extraction of residual PCB from the retrofilled transformer.